Process for making silver protective cloth



Patented Sept. 6, 1949 PROCESS FOR MAKING SILVER PROTEC- TIVE CL ElwoodC. Jackson and Tully H. Babb, Lyman, S. 0., asslgnors to Pacific Mills,a corporation of Massachusetts No Drawing. Application February 11,1948, Serial No. 7,704

This invention relates to cloth for covering silverware to preventtarnishing and more particularly to an improved process of treatment ofthe cloth to impart to it the protective properties.

In the prior processes of this type the cloth has been treated withvarious silver compounds which have an aflinity for and absorb thetarnishing gases and thereby protect the silverware wrapped in thetreated cloth. One of the best of these prior processes is disclosed inBarnard Patent No. 2,003,333 issued June 4, 1935. In the process of thatpatent the silver protective cloth is prepared by a two-bath or two-stepprocedure. In the first bath or treating step the cloth is impregnatedwith aqueous silver nitrate solution and the excess solution squeezedout. In the second bath or step the clothis impregnated with an aqueoussolution of sodium ferrocyanide, and these two chemicals are then causedto react, in situ, on the cloth, so as to produce silver ferrocyanidewhich is the active chemical compound for preventing tarnishing. In thispatented process locust bean gum is used as a protective colloid ordispersing agent for the sodium ferrocyanide solution. I

In addition to the disadvantage of the twobath or two-step featurerequired by this patented process, it is characterized bya tendency toproduce dark streaks in the treated cloth. .Also. it is subject to lossof silver salts caused by the silver dissolving out in the second bathand precipitating in the solution instead of on the fabric.

The process of the present invention constitutes a substantialimprovement over the above prior process in that it requires only aone-bath or onestep procedure, substantially eliminates the loss ofsilver salts, and does not substantially discolor or streak the cloth.In accordance with this new process, the silver ferrocyanideis notformed, in

situ, on the cloth but instead is produced prior to application to thecloth.

One of the important features of this new process is that the silverferrccyanide is precipitated in extremely fine particle size and thisfinely divided precipitate is applied directly to the cloth, and adheresstrongly thereto. There is no opportunity for the silver salts to bedissolved out in the second bath, in contrast to the above Barnardprocess, because the salts are precipitated preliminarily in thesolution rather than on the fabric and the fabric is treated only once.

A commerciall advantageous procedure for precipitating the silverferrocyanide is to carry out the precipitation in an aqueous medium anduse in this medium a dilute solution of locust bean divided precipitate,which is stable as to particle size. A typical but nonlimiting exampleof the constituents and procedure used for preparing this finely dividedsilver ferrocyanide precipitate 5 is as follows:

A. Cooked and cooled 6 lbs. of locust bean gum per 120 gal. water.

B. In 60 gallons of solution A dissolved 59 lbs, of silver nitrate.

C. In 60 gallons of solution A dissolved 46 lbs. of sodium ferrocyanide.

In preparing the precipitated silver ferrocyanide from the aboveconstituents it was found that a critical procedure was involved; thatis, the solution produced in C above must be added to the solutionproduced in B above to obtain the fine particleprecipitate. When thereverse order of mixing these two solutions was used, that is, whensolution "B" was added to solution C, undesirable large particles wereobtained in the precipitate. Accordingly, in the preferred process,solution "3 was first poured into a wooden tub and agitated vigorouslyby means of a propellor-type mixer. Next, solution "C was slowly pouredinto the tub. At first the mixture became very thick but toward the endof the addition it thinned out. After all of solution C had been addedto solution B" the final mixture was further agitated by means of anhomogenizer, or other high speed mixer, or alternatively the mixture waspassed several times through a colloid mill. After the completion ofthis homogenizing treatment the final mixture had a consistency of andlooked like a thin cream. This cream was of a uniform finely dispersednature and free of large particles. This final mixture contained thereaction products 0 silver ferrocyanide and sodium nitrate.

Although it is not considered essential, we may use in the abovecomposition a small amount of 40 alkali, e. g. sodium carbonate, to keepthe solution on the alkaline side and thereby prevent the possibility offree hydrogen cyanide gas being liberated in the after-use of thetreated cloth. The amount of alkali, such as sodium carbonate, which 4may be added may be the theoretical amount (or slight excess) needed toreact with any such liberated gas to convert it to the sodium salt. Theprocedure for preparing the composition utilizing the alkali issubstantially the same as described above but the amounts of therespective constituents vary slightly because of the addition of thealkali, as shown by the following illustration:

A. Cooked and cooled 5-6 lbs. locust bean gum per gal. water.

gum thatassures the formation of a very finely 66 B. In 49 gal. ofsolution A dissolved 59 lbs. of

aeaaeac silver nitrate. to this.

C. In 60 gal. of solution A dissolved 43-45 lbs. of sodium ferrocyanide.

D. In 7 gal. of water disssolved 18 lbs. of sodium carbonate.

Here again it was important that the solutions be mixed in the ordergiven to obtain the fine particle precipitate of silver ferrocyanide.

The ammonia used in the modified composition described just about servesthe purpose of preventing any increase in particle size of thepreciptate caused by the presence of the sodium carbonate. It was foundthat sodium carbonate produced sonic increase in the particle size ofthe precipitate and caused the precipitate to settle out too quickly. Byadding ammonia to the silver nitrate before precipitation it was foundthat this settling-out tendency was eliminated entirely and theresulting suspension would stand for several days with only very slightsettling. This function of the ammonia in producing a better dispersionappears to be quite unique, and although it causes some small amount ofthe silver to precipitate as brownish silver oxide when the sodiumferrocyanide was added, this brown color disappeared and the whitesilver ferrocyanlde was formed. In the drying of the cloth on which thiscomposition is used the ammonia is liberated by the heat.

For application of either of the above compositions to cloth to impartto it silver protective properties, any suitable type of textiletreating equipment which is commonly used for applying treatingsolutions to cloth may be employed. For example, the composition may bepadded onto the cloth in a conventional pad using either one or two dipsof the cloth in the composition and either single or double squeezingoperations on the padded cloth. After the composition is padded on thecloth the treated cloth is passed to a suitable type of drying equipmentsuch as a conventional tenter frame equipped with hot air drying wherethe cloth is air dried while being stretched to the desired width.

Cloth treated in this manner has excellent silver protective propertiesand may be used for silver-wrap and like purposes. The finelyprecipitated silver ferrocyanide which remains in this treated clothwill absorb or take up tarnishing gases to which the cloth is subjected,and in that way these gases are prevented from penetrating the cloth andtarnishing the silver wrapped within the cloth. When using the abovetypical formulations and assuming approximately 90% pick-up of thetreating solution on the cloth in the padding operation, the amount ofsilver introduced into the cloth is approximately 3.4% based on theweight of the cloth. While these amounts are representative of goodcommercial practice it is to be understood that they can be lowered orraised as desired.

The amount of sodium ferrocyanide specified above is slightly greaterthan the theoretical amount, i. e. 42 pounds, required to react with thesilver nitrate. This excess is used for the purpose of assuring completereaction but the amount may be varied, as desired, from the theoreticalamount to any suitable excess. An excess of some proportion is usuallydesirable to make sure that all the silver is precipitated as silverferrocyanide and thereby eliminate any opportunity of black streaksdeveloping in the cloth upon drying due to any unreacted silver nitrate.

Added 4 gal. of ammonia (25%) a Although We have found the abovementioned locust bean gum to be commercially advantageous and we haveobtained very good results with this gum, it is to be understood thatthis invention contemplates the use of other suitable gums, such as forexample gum karaya, gum tragacanth and gum arabic, which will functionsatisfactorily. The locust bean gum serves initially as a precipitatingmedium for the silver nitrate and sodium ferrocyanide reactants andassures the formation of an extremely fine precipitate when these twochemicals are reacted in the aqueous locust bean gum solution. This gumalso functions to some extent to bind the reaction product, i. e.,silver ferrocyanide. in the fabric; although the very fine particles ofthis product are probably partially held by entrapping action of thefibers.

Various modifications and changes may be made in the above-describedproducts and procedures for carrying out the process of this inventionso long as the characteristic extremely fine precipitate of silverferrocyanide is produced and the composition does not have anydiscoloring or other adverse effects on the cloth.

What is claimed is:

1. In a process of preparing silver protective cloth in which silvernitrate is reacted with sodium ferrocyanide to produce silverferrocyanide, the improvement of carrying out this reaction in aqueoussolution preliminarily to application of the composition to the cloth soas to form the silver ferrocyanide beforehand and-thereby enable depositof the silver ferrocyanide in the cloth in a F one-bath procedure andwithout staining or discoloring the cloth.

2. In a process of preparing silver protective cloth in which silvernitrate is reacted with sodium ferrocyanide to produce silverferrocyanide, the improvement of dissolving the silver nitrate in anaqueous solution of locust bean gum, separately dissolving the sodiumferrocyanide in an aqueous solution of locust bean gum, mixing the twoseparately prepared solutions to effect reaction therebetween with theproduction of silver ferrocyanide in finely dispersed particle-sizeform, and applying this fine precipitate of silver ferrocyanide to thecloth to impart silver protective properties thereto.

3. In the production of silver ferrocyanide composition for treatment ofcloth to impart silver protective properties thereto, the improvement ofprecipitating the silver ferrocyanide in an extremely fine particle sizewhich is especially adapted for uniform application to the cloth andwithout substantial discoloration of the cloth, said precipitation beingcarried out by reacting an aqueous solution of silver nitrate and aseparately prepared aqueous solution of sodium ferrocyanide in thepresence of locust bean gum which effects formation of minimum sizeparticles, in the precipitated silver ferrocyanide.

4. The process of preparing a composition suitable for treatment ofcloth to impart silver protective properties thereto comprisingpreparing separate aqueous solutions of silver nitrate and sodiumferrocyanide each dissolved in locust bean gum solution, and reactingsaid solutions together so as to produce an extremely finely dividedprecipitate of silver ferrocyanide.

5. The process of preparing a composition suitable for treatment ofcloth to impart silver protective properties thereto comprisingpreparing an aqueous cooked solution of locust bean gum, adding to anamount of this solution an amount equal amount oi the locust bean gumsolution an amount of sodium ierrocyanide, adding to this silver nitratesolution the sodium terrocyanide solution and producing by this order ofaddition a finely divided precipitate or silver ferrocyanide which issuitable for application to cloth by padding.

6. The process of preparing a composition for treating cloth to impartsilver protective properties thereto comprising preparing first anaqueouscooked solution of locust bean gum, dissolving in a portion 01'this aqueous solution an amount of silver nitrate and adding ammonia tothis solution, then dissolving in another portion of said aqueous cookedsolution 01 locust bean gum a suitable amount of sodium ferrocyanide.next slowly adding to the locust bean gum solution of silver nitrate andammonia the said solution or sodium ierrocyanide dissolved in locustbean gum, agitating the resulting mixture and flnally adding thereto anamount of sodium carbonate sufllcient to neutralize any free hydrogencyanide which might be liberated irom the composition or in itsafter-use on cloth.

7. A process of preparing a composition suitable for application tocloth to impart silver protective properties thereto comprisingdissolving silver nitrate in an aqueous solution of locust bean gum,dissolving in another portion of aqueous solution of locust bean gum,sodium ferrocyamde in sufllcient amount to react completely with thesilver nitrate, mixing these two solutions so as to precipitate in afinely divided form silver ferrocyanide, and subjecting this mixture toan homogenizing treatment which effects a' fine uniform dispersion ofall of the constituents.

' ELWOOD C. JACKSON.

TULLY H. BABB.

No references cited.

